(a) Field of the Invention
The present invention relates to a semiconductor device including a MIS (Metal-Insulator-Semiconductor) transistor, and more particularly, to the structures of a semiconductor device including a MIS transistor having a gate insulating film, which is capable of preventing impurities in an electrode from penetrating therethrough to degrade the transistor characteristics.
The present invention also relates to a method for manufacturing such a semiconductor device.
(b) Description of the Related Art
Along with reduction of the dimensions of semiconductor devices, gate insulating films have been rapidly reduced in thickness, and there is a need for an ultra thin gate insulating film having a thickness of 3 nm or less. However, a conventional silicon oxide film used for a gate insulating film and having such a small thickness suffers from the following phenomena or problems. Specifically, impurity (boron) ions in a gate electrode are thermally diffused and penetrate through the silicon oxide film to a silicon substrate. Meanwhile, a so-called direct tunneling effect is also caused by which electrons penetrate through the insulating film based on the quantum mechanical theory. These phenomena increase gate leakage current.
As a method of solving the problem of penetration through the silicon oxide film by the boron from the gate electrode, the use of a silicon oxynitride film produced by introducing nitrogen into a silicon oxide film has been proposed. As a conventional method of forming a silicon oxynitride film, formation of a gate insulating film by directly thermally oxynitriding a surface of a semiconductor substrate is known.
Many proposed methods use a rapid thermal treatment system to form an oxynitride film for a MOS (Metal Oxide Semiconductor) transistor. For example, an oxide film produced on a silicon substrate is thermally nitrided to form a silicon oxynitride film. Alternatively, a nitride film produced on a silicon substrate is thermally oxidized to form a silicon oxynitride film.
In the above processes, when oxidation is conducted to a nitride film directly formed on a silicon substrate, the nitrogen peak is generally positioned at the interface between the oxynitride film and the silicon substrate, and the interface state increases as a result. Therefore, normally, an oxide film is first formed and the film is then nitrided. For example, according to a method described by Japanese Patent Laid-Open Publication No. Hei. 2-256274, a thermally oxidized film is formed on a surface of a silicon substrate by pyrogenic oxidation, or dry oxygen oxidation, and then the film is allowed to thermally react with nitrogen in a nitriding gas ambient. Thus, nitrogen is introduced into the silicon oxide film. Herein, the nitriding gas may be a nitrogen gas, an ammonia gas, a nitrous oxide gas or a nitrogen monoxide gas
For example, Japanese Patent Laid-Open Publication No. Hei. 6-140392 describes nitriding using plasma in order to solve the disadvantage associated with the above described thermal nitriding. In the plasma treatment, a semiconductor wafer having a silicon oxide film as thick as 4.0 nm formed on a silicon substrate is transported into a vacuum chamber, and heated up to temperatures in the range from 700xc2x0 C. to 900xc2x0 C. using a rapid thermal treatment system. Then, an ammonia gas is introduced as a nitriding gas, and a vacuum ultraviolet beam by Ar plasma generated at a vacuum ultraviolet plasma light emitting disc lamp is irradiated upon the wafer surface.
In the above plasma treatment, the ammonia gas is photodisintegrated by photo excitation, and the silicon oxide film is directly nitrided using resulting much reactive, high energy active nitrogen, so that a silicon oxynitride film is formed. In recent years, control of the nitrogen profile in a silicon oxynitride film has been recognized as an indispensable technique in forming a gate oxide film with superior electric characteristics.
Journal of Applied Physics, Vol.84, page 2980 (J. Appl. Phys. 84 (1998) 2980) describes a method of controlling the nitrogen position in such a gate oxynitride film by using a rapid thermal treatment system. More specifically, treatments with nitrogen monoxide, oxygen, and nitrogen monoxide are sequentially performed in this order, resulting in that the nitrogen in the gate oxynitride film is localized at both the interface and the surface of the gate oxynitride film having a thickness of 4.0 nm. In this structure, hot carrier resistance in the MOS transistor is improved by the nitrogen positioned at the interface. In addition, boron ions in the gate electrode can be prevented from penetrating to the silicon substrate by the nitrogen positioned at the surface.
In Material Research Society 1999, page 84 (MRS 1999 Spring Meeting Abstract 84), for improvement of the charge mobility in the channel region of a MOS transistor, the nitrogen position in the gate insulating film is allowed to reside at the center of the gate insulating film.
Meanwhile, it is reported that, in view of the function of controlling the gate leakage phenomenon, a silicon oxynitride film is equivalent to a silicon oxide film. In contrast, it has been reported that the use of high dielectric constant film other than a silicon oxide film, such as a high dielectric constant metal oxide film including an aluminum oxide film having a dielectric constant of 7 to 9 and a zirconium oxide film having a dielectric constant of 10, can reduce the gate leakage current more than the use of a silicon oxide film.
However, a high dielectric constant film other than the silicon oxide film suffers from significant disadvantages such as mismatching between a gate electrode and a polysilicon material, and degradation in thermal stability and heat resistance, which lead to degradation in the transistor characteristics. These are significant disadvantages in using the film in practice. Therefore, it is extremely important in next-generation fine transistors to reduce gate leakage current in a silicon-oxide-based insulating film having a thickness of 3.0 nm or less.
The silicon oxynitride film as described above is encountered with the following problems. Firstly, the dielectric constant of an ultra thin, silicon oxynitride film as thin as 3.0 nm or less cannot be higher than that of a silicon oxide film. A silicon oxynitride film, if it is possible for the silicon oxynitride film to have a higher dielectric constant, may have a physical film thickness as large as possible for a fixed, smaller electrical film thickness similarly to other high dielectric constant films. In this case, gate leakage current can be reduced without degrading the transistor characteristics in such an ultra thin film having a thickness of 3.0 nm or less.
Secondly, the nitrogen profile in the silicon oxynitride film can hardly be controlled. For example, when the control method described in Journal of Applied Physics, vol.84, page 2980 is employed, the use of a nitrogen monoxide gas prevents the nitrogen amount from being reduced at the silicon substrate interface. According to the nitriding method described in Material Research Society 1999, Spring Meeting, Abstract, page 84, a high pressure gas ambient at 25 atm., for example, is necessary to set the nitrogen position in the center of the film, which is not suitable for mass production type devices however. A high temperature, thermal nitriding reaction using a nitrogen monoxide gas and an oxygen gas is encountered, and therefore the control of the nitrogen position should be even harder.
Thirdly, the nitriding reaction is caused in the interface, which increases the roughness of the interface as the nitrogen amount increases. According to the method described in Japanese Patent Laid-Open Publication No. Hei. 6-140392 in particular, the roughness of the interface increases in the process of nitriding the oxide film.
The present invention is directed to a solution to the above described disadvantages. More specifically, it is an object of the present invention to provide a semiconductor device and a MIS device, which is capable of preventing impurity ions in an electrode adjacent to the insulating film from penetrating through the insulating film to degrade the opposite electrode or substrate, and thereby providing a semiconductor device or a MIS device having excellent transistor characteristics.
It is another object of the present invention to provide a method of manufacturing such a simiconductor device of a MIS device.
The present invention provides, in a first aspect thereof, a semiconductor device including an active element having a silicon oxynitride film including silicon nitride and silicon oxide as main components thereof, the silicon oxynitride film having a first specific dielectric constant which is larger than a second specific dielectric constant theoretically calculated from a weighted average of a specific dielectric constant of silicon oxide and a specific dielectric constant of silicon nitride, the weighted average being based on a weight ratio between the silicon nitride and the silicon oxide in the silicon oxynitride film.
The present invention provides, in a second aspect thereof, a method for forming a semiconductor device having a MIS transistor including the steps of:
forming source/drain/channel regions of the MIS transistor on a silicon substrate;
forming a silicon oxide film on the silicon substrate in association with the source/drain/channel regions by using active oxygen; and
nitriding the silicon oxide film by using active nitrogen to form a silicon oxynitride film as a gate insulating film of the MIS transistor, the silicon oxide film forming step and the nitriding step being conducted continuously in a single chamber while controlling a pressure inside the single chamber and electric power so that the silicon oxynitride film has a specific dielectric constant which is larger than an expected specific dielectric constant theoretically calculated from an amount of the active nitrogen used.
The present invention provides, in a third aspect thereof, a method for forming a semiconductor device having a MIS transistor including the steps of:
forming source/drain/channel regions of the MIS transistor on a silicon substrate; and
forming a silicon oxynitride film on the silicon substrate in association with the source/drain/channel regions by using active oxygen and active nitrogen;
the silicon oxynitride film forming step being conducted while controlling a pressure inside a chamber and electric power so that the silicon oxynitride film has a specific dielectric constant which is larger than an expected specific dielectric constant theoretically calculated from an amount of the active nitrogen used.
The present invention provides, in a fourth aspect thereof, a method for forming a semiconductor device having a MIS transistor including the steps of:
forming source/drain/channel regions of the MIS transistor on a silicon substrate;
forming a silicon nitride film on the silicon substrate in association with the source/drain/channel regions by using active nitrogen; and
oxidizing the silicon nitride film by using active oxygen to form a silicon oxynitride film as a gate insulating film of the MIS transistor;
the silicon nitride film forming step and the oxidizing step being conducted continuously in a single chamber while controlling a pressure inside the single chamber and electric power so that the silicon oxynitride film has a specific dielectric constant which is larger than an expected specific dielectric constant theoretically calculated from an amount of the active nitrogen used.
In accordance with the semiconductor device of the present invention and the semiconductor device manufactured by the method of the present invention, the silicon oxynitride film has a larger specific dielectric constant, which is larger than the expected specific dielectric constant theoretically calculated from the composition of the oxynitride film. Thus, the silicon oxynitride film has a smaller physical thickness compared to the equivalent oxide-film thickness, and has a larger function for preventing impurities in the overlying layer or gate electrode of the MIS transistor from penetrating therethrough toward the underlying layer or the silicon substrate.
The term xe2x80x9cequivalent oxide-film thicknessxe2x80x9d of a subject insulating film as used herein means the thickness of a specific silicon oxide film having an electric property (or capacitance characteristic) equivalent to the electric property of the subject insulating film. The electric property of a film is typically measured by a capacitance it affords when it is used as a capacitor insulator film. On the other hand, the physical thickness of the insulating film can be measured by an electron microscope or ellipsometer. The physical thickness is generally referred to as simply xe2x80x9cthicknessxe2x80x9d.